This invention relates generally to gas turbine engines and more particularly to a fuel nozzle for supplying fuel to the combustor of such engines.
A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and ignited for generating hot combustion gases. These gases flow downstream to one or more turbines that extract energy therefrom to power the compressor and provide useful work such as powering an aircraft in flight. In combustors used with aircraft engines, the fuel is typically supplied to the combustor through a fuel nozzle positioned at one end of the combustion zone, and air is supplied through a surrounding assembly, known as a swirler, which imparts a swirling motion to the air so as to cause the air and fuel to be thoroughly mixed.
Because the fuel nozzle is located in the compressor discharge gas stream, it is exposed to relatively high temperatures. The presence of high temperatures around the fuel nozzle can cause the fuel passing through an inner passageway of the fuel nozzle to form granules of carbon on the walls of the inner passageway. The carbon or coke formation on the walls of the inner passageway may cause the fuel nozzle to become clogged. Excessive temperatures can also cause the fuel in the fuel nozzle to gum up, thereby further causing the fuel nozzle to become clogged. In addition, if the fuel becomes overheated, it may begin to vaporize in the inner passageway, thereby resulting in intermittent or non-continuous fuel delivery to the combustor.
Consequently, conventional fuel nozzles typically include a heat shield in the form of a tubular housing that surrounds a fuel tube so as to define an annular air gap therebetween. The air gap serves as a thermal barrier to protect the fuel in the fuel tube against coking. The fuel nozzle is bent at about 90 degrees at a point between its two ends to permit its installation into the combustor.
During engine operation, the temperature of the housing is greater than the temperature of the fuel tube resulting in differential thermal expansion. The housing is mounted to the fuel tube in conventional fuel nozzles by a fixed joint at one end of the fuel tube for dynamic stability and a sliding joint at the other end of the fuel tube, opposite the bend, to accommodate the thermal expansion differential. The sliding joint typically includes an O-ring boss on the end of the fuel tube that slides inside a seat formed on the housing.
Because the fuel nozzle is located in the compressor discharge gas stream, the thermal expansion differential is most acute during engine acceleration, such as at take off. The housing quickly outgrows the fuel tube, resulting in the O-ring boss being pulled toward the end of the seat. Conversely during engine deceleration, the housing cools (and hence shrinks) more quickly than the fuel tube, causing the O-ring boss to be pushed back farther into the seat. However, because of the bend in the fuel nozzle, a portion of the nozzle lies at a 90 degree angle with the longitudinal axis of the seat. Relative thermal expansion in this offset portion forces the O-ring boss out of axial alignment with the seat (i.e., they are no longer co-axial). As the housing shrinks and tries to push the O-ring boss back into the seat, it cannot because the O-ring boss binds within the seat due to the misalignment caused by the offset. This binding causes high compressive stress, yielding and foreshortening of the fuel tube. As repeated cycles of acceleration and deceleration continue to foreshorten the fuel tube through this xe2x80x9cthermal ratchetingxe2x80x9d mechanism, the boss can become fully extracted from the seat.
The resulting fuel leakage into the air gap, coking and indeterminate dynamic behavior of the unseated fuel tube can eventually result in fuel nozzle failure.
Accordingly, there is a need for a fuel nozzle that is capable of withstanding high compressor discharge temperatures while avoiding the thermally induced binding and stress problems of prior nozzles.
The above-mentioned needs are met by the present invention which provides a fuel nozzle having a fuel tube enclosed by a housing. The fuel tube and housing are connected by a movable joint including a tubular seat formed on the housing and a boss formed on the fuel tube so as to be disposed within the tubular seat. The boss includes a contact surface that is in sliding engagement with the tubular seat and is rotatable about an axis that is perpendicular to the seat""s longitudinal axis. The fuel nozzle further includes a fixed joint that connects the housing to the same section of the fuel tube as the movable joint.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.